As functions of a screw, there are a “fastening function” of fastening an object and a “feed function” of feeding an object. Both of the functions achieve maximum effects if both flanks of a male screw M and a female screw F come into face contact with each other, as shown in FIG. 5(a). Meanwhile, there is a tolerance in sizes of an actual male screw and an actual female screw. Moreover, there is also a tolerance in sizes of tools and machine tools for forming those screws. Thus, both flanks of the screws are very unlikely to come into face contact with each other. Even in screws fitted nicely into each other, technically speaking, as shown in FIG. 5(b), the both flanks of the male and female screws M and F only come into point contact with each other in spots indicated by the symbols P in FIG. 5(b). A strong axial stress acting in fastening merely causes a state close to surface contact due to elastic deformation of a screw material. Therefore, mostly, as shown in FIG. 5(c), the male and female screws are loosely fitted into each other.
However, the point contact state and the state where the screws are loosely fitted into each other as described above are far from good as machine parts. Thus, the inventors of the present application have investigated the causes of the point contact state and the state where the screws are loosely fitted into each other. As a result, it has been found out that there are three main causes, including inaccuracy of feeding a tap, a loss of shape in a tap cutting edge, and run-out of the tap in cutting. As to feeding of the tap, a recent high-precision machine tool such as a NC machine tool enables accurate feeding by feeding forward the tap by the machine tool in synchronization with rotation. Thus, it has been found out that, if the problems of the loss of shape in the tap cutting edge and the run-out of the tap in cutting are resolved, it is possible to realize a tap capable of performing highly accurate female screw forming so as to achieve a surface contact for a long period of time.
Meanwhile, as to a shape of a bevel lead 2a of a screw part 2 in a conventional tap 1 as shown in FIG. 6, the portion B of the bevel lead 2a in FIG. 6 will be shown in closeup in FIGS. 7(a) and 7(b). First, a shape of a male screw having a crest face 2e lower than a complete thread is formed. At the same time, as shown in FIG. 7(a), a cutting face 4a is formed by a flute 4. Next, as shown in FIG. 7(b), honing is performed along a cutting edge E that is a nodal line between the cutting face 4a, a following flank 2c of the thread, a leading flank 2d thereof (not shown in FIGS. 7(a) and 7(b)) and the crest face 2e, and along a ridgeline R between the following flank 2c and the leading flank 2d of the thread and the crest face 2e. Accordingly, the bevel lead 2a has its whole shape slightly rounded, including the cutting edge E and the ridgeline R.
Therefore, in the conventional tap 1, at the time of cutting and forming a female screw, a very large cutting load is applied to a cutting edge corner part where the cutting edge E and the ridgeline R intersect with each other. Accordingly, a temperature of the cutting edge corner part is increased, and a female screw material is likely to be welded thereto. Thus, a built-up edge is repeatedly attached and detached in the corner part. As a result, wear and fracture and thus a loss of shape in the cutting edge occur. In order to prevent the above problems, a surface of the screw part is often coated with a hard layer which is highly resistant to welding. However, there is a problem that even the hard layer wears and fractures.
Consequently, it is an object of the present invention to provide a tap capable of smoothly performing highly accurate female screw forming at high speed by resolving the problems of the loss of shape in the tap cutting edge and the waggling of the tap in cutting.